CALCULATION OF HIGH ROTATION ENERGIES OF THE WATER MOLECULE USING THEGENERATING FUNCTION MODEL

The previously developed generating function model is applied in this paper to fitting high rotational levels of the water molecule in the d omain of rotational quantum numbers where the standard power series ex pansion of the rotational Hamiltonian has a slow rate of convergence o r even diverges. This model typically provides a considerable improvem ent in the standard deviation of fit with respect both to the conventi onal polynomial Hamiltonian and to the Fade-type Hamiltonian (with the same or fewer number of adjustable parameters). Ground state rotation al levels recovered by Flaud et al. (Mol. Phys. 32, 499-521 (1976)) fr om flame spectra are fitted with accuracy near that of the experimenta l values: weighted standard deviation chi = 1.8 for 422 levels up to J , K-a less than or equal to 20 and chi = 2.1 for all available levels up to J less than or equal to 35. New ground state data reported by To th (J. Opt. Sec. B 8, 2236-2255 (1991)) are fitted up to J, K-a less t han or equal to 10 with RMS = 4 X 10(-5) cm(-1). Tests of extrapolatio n properties of the generating function model are discussed. In certai n cases the accuracy of extrapolation was better than the accuracy of fitting with the standard rotational Hamiltonian. (C) 1995 Academic Pr ess, Inc.